{"error":"Content-Type header [application/x-www-form-urlencoded] is not supported","status":406}

In order to fix issue specify header with your curl XPUT.

Original Post

| exmple

curl -XPUT 'localhost:9200/_settings' -H 'Content-Type: application/json' -d '
{
    "index" : {
        "number_of_replicas" : 0
    }
}'